Field
Various features relate to integrated circuits, and more particularly to methods and apparatuses for improved programmable memory cells featuring antifuses.
Background
Integrated circuits are interconnected networks of electrical components fabricated on a common foundation called a substrate. The substrate is typically a wafer of semiconductor material, such as silicon. Various fabrication techniques, such as layering, doping, masking, and etching, are used to build millions of resistors, transistors, and other electrical components on the wafer. The components are then wired together, or interconnected, to define a specific electrical circuit, such as a processor or a memory device.
Fusible elements are employed in integrated circuits to permit changes in the configuration of the integrated circuits after fabrication. For example, fusible elements may be used to replace defective circuits with redundant circuits. As another example, fusible elements may be used to create one time programmable (OTP) or multi-time programmable (MTP) memory circuits. Individual memory cells of an OTP memory cell may be written to once in order to create read only memory modules that cannot be easily altered and/or are secure.
One type of fusible element is a metal fuse. The metal fuse is composed of a metal alloy or metal, such as copper, that may change its state from a conductive, closed circuit state to a substantially non-conductive, open circuit state if a sufficient amount of current flows through the metal fuse. Metal fuses have several disadvantages. For example, the current needed to program the fuse (i.e., blow the fuse to change it from a closed circuit state to an open circuit state) is relatively high. Generating this current consumes a substantial amount of power, particularly for mobile devices where power consumption is a concern. Moreover, relatively large transistors (i.e., transistors having a large chip area) are required to generate the current drive needed to blow the metal fuses. Furthermore, the integrated circuit package having the metal fuses may require a dedicated power pin to handle the high current used for programming the metal fuses. Additionally, metal fuses provide poor security because the blown fuses may, in some cases, be seen optically. Also, metal fuses offer poor reliability and in some cases may require serial programming.
Another type of fusible element is a gate dielectric antifuse. An antifuse comprises two conductive terminals separated by an insulator or a dielectric, and is fabricated as an open circuit. The antifuse is programmed by applying a high voltage across its terminals to rupture the insulator and form an electrical path between the terminals. Typical prior art gate dielectric antifuses used for programmable memory cells require a high voltage to change the state of the antifuse from an open circuit state to a closed circuit state. The voltage needed to cause the state change is generated using a charge pump. However, charge pumps consume a substantial amount of the integrated circuit's active chip area that may otherwise be used for other active components, such as memory cells.
Therefore, there exists a need for integrated circuits, such as OTP and MTP memory cells, that feature fusible elements that do not suffer from the disadvantages described above in connection with metal fuses and gate dielectric fuses.
There also exists a need for integrated circuits having antifuses that can transition from open circuit states to closed circuit states at lower programming voltages.